1. Field of the Invention
The invention pertains to silicate cements of the type disclosed in my U.S. Pat. No. 4,030,939 patented June 21, 1977.
2. Description of the Prior Art
Silicate cements containing measurable concentration of sodium in reactive form have been shown to be attacked by sulfate ion with the formation of Glauber's salt--an expansive, disruptive formation. Prior Art reports the effectiveness of various phosphate salts as inhibitors of this formation. This invention extends that art with the addition of a discovery that urea and sodium acetate surpass in performance the behavior of sodium and potassium pyrophosphates, sodium and potassium tripolyphosphates, sodium and potassium hypophosphites, and the metaphosphates. Sodium acetate and urea have been found to combine superior adhesive properties, low permeability, moisture retention (humectant) properties and superior sulfate ion resistance compared to the phosphates.
In general, the silicate cements of this invention exhibit optimum adhesion to glass, metal, ceramics, wood and portland concrete. However, they can also be induced to exhibit affinity for organic and plastic surfaces by modification with sodium tripolyphosphate. This salt at low concentrations (1-2%) will convert the adhesive properties from polar affinity to nonpolar affinity. With this phosphate, the cement surprisingly exhibits adhesion to plastics and organics such as asphalt, rubber, polyvinyl chloride, and the polyolefins. This phenomena is amenable to control such that the cement, by judicious addition of this phosphate, can be made to bond to metals and minerals or to plastics and organics or to both in varied degree. As the adhesion to metals and minerals is increased, the adhesion to plastics and organics is diminished and vice versa.
Most acid-resistant silicate cements employed in grouting, guniting, or similar applications require several hours of curing history to achieve a load bearing or rigid state. Thixotropic agents are therefore required to assure retention of coatings with no slump or drainage. Prior art has offered various clays, cellulosic and polysacchride additives such as starches, alginates, and similar colloidal additives to impart desired rheological qualities. This invention extends this practice to include or substitute for the above such chemically rather than physically active agents as to induce a controlled, early degree of viscosity increase by the generation of thickening agents in-situ by conversion of a portion of the alkali metal silicate to such a thickener. Surprisingly, tribasic magnesium phosphate added at 0.25 to 2.5 pph, basis dry solids mixture of cement ingredients, will react at even very low temperatures (32.degree. F.) to produce a silicate salt of magnesium in hydrous, colloidal form such that a thixotropic slurry forms within 5 minutes of addition of water to the powder. Within 10 minutes, a firm gel state is attained, and within 30 minutes a compressive strength of 100-1000 psi is achieved, depending upon temperature and concentration of the magnesium phosphate. This stabilization of the coating can be very useful in applications where vibration or impact by workers and equipment may be imminent. Conventional thixotropic agents such as the bentonites, clays, and organic colloids may be added or omitted by option of the user but are only necessary when the water to solids ratios are uncontrollably higher than optimum, as with certain types of guniting equipment.